fix: prevent segfault in StringIO#seek with SEEK_END on null device #474

Job	Run time
ruby-versions / Generate Ruby versions	4s
ubuntu-24.04 3.3	17s
ubuntu-24.04 3.2	19s
ubuntu-24.04 3.1	14s
ubuntu-24.04 debug	22s
ubuntu-24.04 3.4	17s
ubuntu-24.04 2.7	40s
ubuntu-24.04 3.0	13s
ubuntu-22.04 2.7	41s
ubuntu-24.04 truffleruby-head	21s
ubuntu-24.04 head	18s
ubuntu-22.04 3.4	17s
ubuntu-22.04 3.1	16s
ubuntu-22.04 3.0	13s
ubuntu-22.04 head	17s
ubuntu-22.04 3.2	18s
ubuntu-22.04 3.3	25s
ubuntu-22.04 truffleruby-head	21s
ubuntu-22.04 debug	21s
ubuntu-22.04 jruby-head	1m 1s
ubuntu-24.04 jruby-head	1m 20s
	8m 35s

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